The invention relates to a motor vehicle air spring system having an ultrasonic arrangement for the contactless distance measurement in accordance with the pulse/echo method.
An air spring comprises essentially a flexible member having an upper end closed off by a cover plate and a lower end closed off by a roll-off piston. The cover plate is preferably fixed to the chassis and the roll-off piston is mounted on an axle.
The variable clear distance between the cover plate and the roll-off piston within the air spring is measured with the aid of ultrasonic pulses for determining the air spring height. The method most used is the pulse/echo method. Here, an ultrasonic signal is propagated from an ultrasonic converter attached to the cover. The ultrasonic signal is reflected by a reflector mounted on the roll-off piston and is received by an ultrasonic converter mounted on the cover and is further transmitted to an evaluation electronic unit for evaluation.
The distance passed through by the ultrasonic signal is computed from the running time of the signal and the speed of sound. From this distance, the spring height can be computed.
Alternatively, the ultrasonic converter can be mounted on the roll-off piston in lieu of on the cover. The reflector would then have to be mounted on the cover in lieu of on the roll-off piston.
The sonic speed needed for computing the distance is dependent upon temperature. Measuring methods with additional reference paths are suggested in order to avoid a complicated temperature compensation when determining the height of the air spring.
It is necessary to provide the ultrasonic converter, which is configured as a transmitter/receiver, with an elastic or damped suspension in order to avoid a propagation of body sound within the air spring components. This has the consequence that the ultrasonic converter moves backward somewhat when there is a pressure increase whereby the distance to the reference reflector fixedly mounted to the housing is correspondingly increased which, in turn, has the consequence that the measurement value is made erroneous.
German patent publication 3,620,957 discloses such an air spring having an ultrasonic pulse/echo system for measuring height. An additional fixed target is suggested in order to preclude the effects of running-speed changes of the pulses which can occur because of changes in air pressure, temperature and humidity in the interior of the flexible member. This additional fixed target is a reflector in the form of a planar metal disc attached to a stem-like rod. The reflector is arranged spatially fixed in front of the ultrasonic transmitter.
With the above, various disadvantages result. The fixed target comprising the stem-like rod and the metal disc is a mechanically sensitive component. Because the fixed target has a certain structural elevation, the clear elevation of the air spring is reduced and therefore also the maximum possible deflection path of the air spring. On the other hand, the needed structural elevation required for the air spring is increased. A further disadvantage is that the reflector is mounted relatively close to the ultrasonic transmitter and, in this way, the useful signal, which is received over the additional distance, is drowned out by the reference signal possibly to the extent of unrecognizability.
In view of the foregoing, it is an object of the invention to provide an air spring system for a motor vehicle having an ultrasonic arrangement for the contactless distance measurement in accordance with the pulse/echo method which ameliorates the above-mentioned disadvantages.
The air spring system of the invention is for a motor vehicle and includes: a first connecting part defining a cover plate; a second connecting part defining a roll-off piston; a flexible member having a first end connected to the first connecting part and a second end connected to the second connecting part; an ultrasonic arrangement for making contactless measurements of the distance between the first and second connecting parts; the ultrasonic arrangement including an ultrasonic transmitter/receiver mounted on one of the connecting parts and a reflector mounted on the other one of the connecting parts; the reflector being configured as a two-step reflector having first and second steps; and, one of the steps being a target reflector and the other one of the steps being a reference reflector.
The above air spring system comprises essentially that a reference path is provided not fixed in position relative to the transmitter but fixed in position relative to the reflector.
The advantages achieved with the invention include the absence of a reference reflector assigned to a sensor, a general simplification of the construction and a reduction of the space needed for components.
Of great significance is the reliability in the evaluation. A reference reflector at a short distance forward of the sonic converter generates an echo in each case. This is under some circumstances so great that it over-controls the electronics so that, in an individual case, an echo from a greater distance cannot be recognized. By utilizing the two-stage reflector according to the invention, measurement and reference signals have comparatively large amplitudes so that measurement difficulties with respect thereto are no longer present. If the reference and target echos are of approximately the same intensity, then this increases the reliability in the evaluation of the running times. A single automatic control is sufficient.
Preferably, the transmitter/receiver unit is disposed on the air spring cover, that is, this unit is preferably fixed to the chassis and the two-stage target reflector is mounted on the roll-off piston. In this way, the connection of the transmitter/receiver unit with the corresponding electronic component is simplified.
A further embodiment of the invention provides that the bumper, which is usually disposed on the roll-off piston, is configured as a two-stage target reflector. No space for components is lost with the configuration of this kind.
The two-stage target reflector can be realized by a central blind bore placed in the bumper.
If the transmitter/receiver is arranged off-center on the cover (for example, because of a centrally arranged carrier), it is understood that the two-stage target reflector can also be mounted off-center or inclined in the direction toward the transmitter/receiver component on or in the bumper.
With experiments carried out, it has been shown that a blind bore having a diameter of 12 mm and a depth of 7.5 mm provides reliably reproducible results.